Human joints are susceptible to degenerative diseases. Arthritis affecting the knee often causes such pain and discomfort that older patients cannot maintain an independent lifestyle. For these patients, total knee arthroplasty provides an opportunity to restore the knee's functions and lost mobility. Today's orthopaedic surgeon has a wealth of options for customizing surgery to a wide range of patient's anatomies and conditions. The knee system elected must meet the requirements for articular stability and anatomic pathways tracking. However, these two requirement are inherently conflicting. Stability is accomplished through articular conformity and maximum contact area while anatomic tracking is realized through unconstrained mobility.
Total knee replacement systems can usually be grouped under two denominations: condylar and stabilized. Each is tailored to individual needs and preferences. Condylar prostheses provide maximum range of motion. Their usage is indicated with patients who do not need compensation for ligamentous instability. Stabilized tibial prostheses have a central stabilizer post. Minimal clearance between the post and the femoral housing provides increased stability in the anterior-posterior and medial-lateral directions and constrains internal-external rotations. Such prostheses are useful when little reliance is placed on the surrounding soft tissues to stabilize the joint. Although both prostheses have analogously shaped bearing surfaces, the condylar prostheses rely mostly upon the surrounding tendons and ligaments to maintain the knee joint's integrity and to impart stability to the knee during movement.
The three-dimensional kinematics of the prosthetic knee are the result of a delicate harmony between component selection, placement, orientation, and ligament balancing. As the knee flexes, ligaments across the knee joint are sequentially applied. The length and orientation of ligaments allow functional motions and yet provide restraints at the limits of such motions. Conversely, slight variations in component placement or orientation may affect the ligaments' operation. Differences in the radii of curvatures and congruency of the articulating profiles can also induce substantial changes in rotation and translation. Complications arise when the mechanics of the ligaments are not in tune with the mechanics of the total knee geometries. For instance, misplacement or misalignment may cause the ligaments to become highly loaded or overly lax within the functional range of motion.
Success in restoring a knee's normal function can be measured by assessing the post-arthroplasty kinematic signature of a specific implant and comparing it to normal gait kinematics. This kinematic profile is revealed as each component's geometry comes into contact. While it is generally simple to measure the spatial motion of the lower extremity post-operatively, it is more difficult to estimate or predict the functional outcome of a surgical implantation before it is actually performed on the patient.
In the prior art, an in vitro technique for estimating the relative three-dimensional in vivo displacements of a particular total knee system is proposed. The current test method requires the use of natural knee specimens, either fresh or embalmed, to quantify the range of motion of a total knee replacement system. This method has numerous drawbacks. The procedure is lengthy and requires considerable surgical skills. The handling of human specimens presents a potential risk from blood borne pathogens. Moreover, a natural knee may not be implanted more than once. Reproducibility is a major issue as considerable variations are to be expected from specimen to specimen or from day to day. Indeed, because the natural knee presents variations in freshness, quality, size, age, and gender, this procedure is not reliable.
The present invention relieves the concerns associated with using cadaveric specimens for assessment of total knee kinematics. A departure from prior testing grounds is signalled by using a synthetic knee in place of a natural knee. The present application is based on the finding that it is possible to replicate the primary constraints of the knee using synthetic materials and appropriate design principles. Since ligaments are major structures which limit knee kinematics, one needs to duplicate their elastic behavior, placement, and orientation, in order to impart a standardized confining pattern. Hence, the synthetic knee acts as a surrogate joint to receive the total knee replacement prior to measuring its kinematic profile. The present invention overcomes hurdles posed by natural knee sample disparities, biohazards, and surgical skills.